Intermodulation (IM) is caused by non-linearities in the radio frequency (RF) path between the transmitter and the receiver in a communication system. These non-linearities may exist within the transmitter amplification and filtering chain, at the transmit antenna, along the propagation path between the transmitter and receiver, within the receiver antenna, or within the receiver downconversion chain. Mathematically, these non-linear signals are described by a power series expansion that describes the output voltage signal, νout(t), in terms of arithmetic powers of the input voltage signal, νin(t),
            v      out        ⁡          (      t      )        =            ∑              k        =        1            ∞        ⁢                  g        k            ⁢                        v          in          k                ⁡                  (          t          )                    
where gk are the voltage gain coefficients corresponding to the k-th order non linearity. gl is the nominal linear voltage gain of the device.
Intermodulation distortion occurs when two or more modulated or unmodulated RF carrier signals interact to create undesired signals on frequencies other than the original carrier signals. For example, if the input signal consists of two unmodulated sinusoids of the form ν(t)=A cos ωAt+B cos ωBt, the k=3 term in the power series expansion above is
                    g        3            ⁡              (                              A            ⁢                                                  ⁢            cos            ⁢                                                  ⁢                          ω              A                        ⁢            t                    +                      B            ⁢                                                  ⁢            cos            ⁢                                                  ⁢                          ω              B                        ⁢            t                          )              3    =            g      3        ⁡          [                                                                                    A                  3                                (                                                                            3                      4                                        ⁢                    cos                    ⁢                                                                                  ⁢                                          ω                      A                                        ⁢                    t                                    +                                                            1                      4                                        ⁢                    cos                    ⁢                                                                                  ⁢                    3                    ⁢                                          ω                      A                                        ⁢                    t                                                  )                            +                                                                                                            A                  2                                ⁢                                  B                  (                                                                                    3                        2                                            ⁢                      cos                      ⁢                                                                                          ⁢                                              ω                        B                                            ⁢                      t                                        +                                                                  3                        4                                            ⁢                                              cos                        ⁡                                                  (                                                                                    2                              ⁢                                                              ω                                A                                                                                      -                                                          ω                              B                                                                                )                                                                    ⁢                      t                                        +                                                                  3                        4                                            ⁢                                              cos                        ⁡                                                  (                                                                                    2                              ⁢                                                              ω                                A                                                                                      +                                                          ω                              B                                                                                )                                                                    ⁢                      t                                                        )                                            +                                                                                                            AB                  2                                (                                                                            3                      2                                        ⁢                    cos                    ⁢                                                                                  ⁢                                          ω                      A                                        ⁢                    t                                    +                                                            3                      4                                        ⁢                                          cos                      ⁡                                              (                                                                              2                            ⁢                                                          ω                              B                                                                                -                                                      ω                            A                                                                          )                                                              ⁢                    t                                    +                                                            3                      4                                        ⁢                                          cos                      ⁡                                              (                                                                              2                            ⁢                                                          ω                              B                                                                                +                                                      ω                            A                                                                          )                                                              ⁢                    t                                                  )                            +                                                                                          B                3                            (                                                                    3                    4                                    ⁢                  cos                  ⁢                                                                          ⁢                                      ω                    B                                    ⁢                  t                                +                                                      1                    4                                    ⁢                  cos                  ⁢                                                                          ⁢                  3                  ⁢                                      ω                    B                                    ⁢                  t                                            )                                          ]      
All of the expansion terms consist of sinusoidal signals at frequencies mωA±nωB, where m+n=k. In some embodiments, the value of k is odd. The value of k is referred to as the order of the intermodulation product.
In addition to the frequency translation effects illustrated in the above equation, practical communication systems also have complex amplitude (and phase) modulations A=A (t) and B=B(t). The time domain product illustrated in the above equation, therefore, gives rise to a spectrum-spreading effect due to the convolution of the corresponding frequency-domain spectra, in accordance with the well-known Convolution Theorem of linear systems. Therefore, the intermodulation products exhibit both frequency offsets and bandwidth expansion relative to the original signals A(t)cos ωAt and B(t)cos ωBt.
In full-duplex communication systems such as Inmarsat satellite services, the potential for interference exists when these undesired intermodulation products fall on the same RF frequencies used by the receiver. Until mid-2005, Inmarsat assigned channels to aeronautical services of any type that were specifically selected to assure that low-order intermodulation products from multi-carrier radios did not fall in the frequency bands of the receiver. Inmarsat has determined that continuing to provide this frequency management function has an adverse effect on its ability to satisfy the communication demands of its users. Therefore, Inmarsat has determined that it will no longer attempt to manage frequency assignments after 2009.
The decision to end frequency management has raised the possibility that relatively low-order intermodulation products from transceiver transmissions in the Inmarsat uplink band (1626.5 MHz-1660.5 MHz) will fall within the Inmarsat receive band of (1525 MHz-1559 MHz). In such a situation, Inmarsat could effectively interfere with itself. To deal with the possibility of interference, “Classic Aero” channels are, and will continue to be, managed to assure that only high order intermodulation products fall in the GLONASS, GPS, and Inmarsat receive bands. These high order intermodulation products have relatively low amplitude compared to the received signals and low order intermodulation products.
In addition, legacy equipment provides transmit-path filtering to protect these bands. However, new Swift Broadband and some classes of Swift 64 channels will not benefit from such frequency management. Hence, Inmarsat has initiated a development program to produce a new, high-technology transmit-path filter that could be used to provide additional protection in new installations providing Swift Broadband and Swift 64. Unfortunately, such filtering does not, and can not protect any of the frequency bands mentioned above from intermodulation products generated after the filter. Therefore, sources of intermodulation products after the filter, such as transmit antenna intermodulation, continue to be of concern.
For the reasons stated above, and for other reasons stated below which will become apparent to those of skill in the art upon reading and understanding the present specification, there is a need in the art for a system and method to control intermodulation interference.